FIELD OF THE INVENTION
The present invention relates to tire beads which are provided at the radially inner ends of the sidewalls of a tire.
A conventional pneumatic tire comprises an annular tread, a pair of sidewalls extending radially inward from opposite ends of the tread, a bead portion that is formed at the radially inner end of each sidewall and which is provided with an annular bead in its interior, and a carcass that is composed of a number of parallel carcass cords made of either an organic or inorganic material and that passes through the tread and the sidewalls in such a way that each end of the carcass is folded towards the outside around the bead to be held in position.
The beads as they are used in an inflated pneumatic tire have the basic capability of supporting the terminal ends of the carcass cords as they tend to expand outwardly under the inflation pressure, to thereby accommodate the tension of the carcass cords. To fulfill these functions, the beads are required to have not only sufficient strength to withstand an applied great tension and also the great rigidity that is necessary to retain the tire on a wheel rim while maintaining a sufficient dimensional precision on the inner periphery of the bead portion to insure good fit to the rim.
In order to satisfy these strength and rigidity requirements, a plurality of high-modulus steel wires have been used as conventional beads.
However, such conventional beads that comprise more than one steel wire have had the problem that their weight accounts for about 5% of the tire weight, which is one of the obstacles to the objective of realizing lighter tires that is gaining increasing importance these days when vehicles are required to have better performance.
Another problem with the conventional beads is that the steel wires which constitute the beads contact the Carcass cords to potentially cause a breakage of the latter.
Thus, with a view to reducing the weight of tires, several proposals have been made that are based on the use of aromatic polyamide cords as bead forming wires. For example, Examined Japanese Patent Publication No. 33244/1981 proposes the use of steel wire cords in combination with aromatic polyamide wire cords since beads that are formed of aromatic polyamide wire cords alone have such low rigidity that the tire is difficult to mount positively on a wheel rim.
Unexamined Published Japanese Patent Application No. 82705/1975 describes a tire in which the beads are formed solely of cords that are made of aromatic polyamide wires. Further, Unexamined Published Japanese Patent Application No. 43010/1981 describes a tire in which the beads are formed of cords that are made of other wire materials than steel such as aromatic polyamides or glass fibers. However, both types of beads are insufficient in strength against compression strain and flexural deformation to be used in practical applications, particularly in truck and bus tires which are to be used with high inflation pressures, and there has been a possibility that the bead's shape will deform during tire manufacture or running.
An object, therefore, of the present invention is to provide a tire bead that uses an aromatic polyamide in part of the bead forming wires to realize a lighter tire.
Another object of the present invention is to provide a tire bead in which the occurrence of broken carcass cords can be prevented.